The invention relates generally to superconducting magnetic coils and methods for manufacturing them. In particular, the invention relates to a wind-and-react process used to produce mechanically robust, high temperature superconducting coils which have high winding densities and are capable of generating large magnetic fields.
The wind-and-react method involves winding the precursor to a superconducting material around a mandrel in order to form a coil, and then processing the coil with high temperatures and an oxidizing environment. The processing method results in the conversion of the precursor material to a desired superconducting material, and in the healing of micro-cracks formed in the precursor during the winding process, thus optimizing the electrical properties of the coil.
Superconducting magnetic coils, like most magnetic coils, are formed by wrapping an insulated conducting material around a mandrel defining the shape of the coil. When the temperature of the coil is sufficiently low that the conductor can exist in a superconducting state, the current-carrying performance of the conductor is markedly increased and large magnetic fields can be generated by the coil.
Certain ceramic materials exhibit superconducting behavior at low temperatures, such as the compound Bi2Sr2Canxe2x88x921CunO2n+4 where n can be either 1, 2, or 3. One material, Bi2Sr2Ca2Cu3O10 (BSCCO (2223)), performs particularly well in device applications because superconductivity and corresponding high current densities are achieved at relatively high temperatures (Tc=115 K). Other oxide superconductors include general Cu-O-based ceramic superconductors, such as members of the rare-earth-copper-oxide family (ie., YBCO), the thallium-barium-calcium-copper-oxide family (ie., TBCCO), the mercury-barium-calcium-copper-oxide family (ie., HgBCCO), and BSCCO compounds containing lead (ie.,(Bi,Pb)2Sr2Ca2Cu3O10).
Insulating materials surrounding the conductor are used to prevent electrical short circuits within the winding of a coil. From a design point of view, the insulation layer must be able to withstand large electric fields (as high as 4xc3x97105 V/cm in some cases) without suffering dielectric breakdown, a phenomenon that leads to electrical cross-talk between neighboring conductors. At the same time insulation layers must be as thin as possible (typically less than 50-150 m) so that the amount of superconducting material in the coil can be maximized.
Using existing conducting and insulating materials, the maximum magnetic field generated by a superconducting coil is ultimately determined by the winding density (defined as the percentage of the volume of the coil occupied by the conductor) and the coil geometry. However, the large tensional forces necessary for high winding densities can leave conductors in highly stressed and/or strained states. The bend strain of a conductor, equal to half the thickness of the conductor divided by the radius of the bend, is often used to quantify the amount of strain imposed on the conductor through coil formation. Many superconducting magnet applications involving high-density conductor windings require conductor bend strains on the order of 0.2%, and in some cases much higher. The critical strain of a conductor is defined as the amount of strain the material can support before experiencing a dramatic decrease in electrical performance. The critical strain value is highly dependent on the formation process used to fabricate the conductor, and is typically between 0.05%-1.0%, depending on the process used. If the bend strain exceeds the critical strain of a conductor, the current-carrying capability of the conductor, and hence the maximum magnetic field generated by a coil, will be decreased significantly. One approach to manufacturing high-performance conductors having desirable mechanical properties involves starting with a precursor to a high temperature superconducting material, typically a ceramic oxide in a powder form. Despite relatively poor mechanical properties and more complex manufacturing processes which requires high temperatures and an oxidizing environment, high temperature superconducting materials are preferred to low temperature superconducting materials for certain applications because they operate at higher ambient temperatures. Oxide powders are packed into a silver tube (chosen because of malleability, inertness, and high electrical conductivity) which is then deformed and reduced in size using standard metallurgical techniques: extrusion, swaging, and drawing are used for axisymmetric reductions resulting in the formation of rods and wires, while rolling and pressing are used for aspected reductions resulting in the formation of tapes and sheets (Sandhage et al., xe2x80x9cCritical Issues in the OPIT Processing of High-Ic BSCCO Superconductorsxe2x80x9d, Journal of Metals 3, 21, 1991).
Following the deformation process, heating and cooling results in the growth and evolution of individual crystalline oxide superconductor grains in the conductor which typically take on a rectangular platelet shape. Further deformation results in a collective alignment of the crystallographic axes of the grains. An iterative heating/deforming schedule unique to the ceramic oxide forms of superconductors is typically carried out until the desired grain size, alignment, and density of the superconducting state are achieved.
Conductors having a single oxide core, classified as mono-filament composite conductors, result from the iterative schedule described above and can have critical strain values as high as 0.1%. Mono-filament composite conductors can be transformed into multi-filament composite conductors using a rebundling fabrication operation involving further reduction in size of the mono-filament composite conductors, and finally concatenation of individual conductors to form a single conductor. Typically, the evolution of cracks in response to bend strains is more likely in mono-filament composite conductors than in multi-filament composite conductors, where critical strain values increase with the number of filaments in the conductor, and can be greater than 1.0%. Other limitations of mono-filament composite conductors include decreases in crack healing ability and oxygen access to the conductor during processing. Furthermore, because mono-filament composite conductors have only a single superconducting region, it is difficult to control the conductor size and shape, and mechanically robust conductors can not be easily fabricated (K. Osamure, et al., Adv. Cryo. Eng., ICMC Supplemental, 38, 875, 1992). Thus, multi-filament composite conductors have desirable mechanical properties, and can be used in coils requiring high winding densities.
One method used to fabricate coils with multi- and mono-filament composite conductors is the react-and-wind process. This method first involves the formation of an insulated composite conductor which is then wound into a coil. In this method, a precursor to a composite conductor is fabricated and placed in a linear geometry, or wrapped loosely around a coil, and placed in a furnace for processing. The precursor can therefore be surrounded by an oxidizing environment during processing, which is necessary for conversion to the desired superconducting state. In the react-and-wind processing method, insulation can be applied after the composite conductor is processed, and materials issues such as the oxygen permeability and thermal decomposition of the insulating layer do not need to be addressed.
In the react-and-wind process, the coil-formation step can, however, result in straining composite conductors in excess of the critical strain value of the conducting filaments. Strain introduced to the conducting portion of the wire during the deformation process can result in micro-crack formation in the ceramic grains, severely degrading the electrical properties of the composite conductor.
Another method used to fabricate magnetic coils with mono-filament composite conductors is the wind-and-react method. In this method, the eventual conducting material is typicallly considered to be a xe2x80x9cprecursorxe2x80x9d until after the final heat treating and oxidation step. Unlike the react-and-wind process, the wind-and-react method as applied to high temperature superconductors requires that the precursor be insulated before coil formation, and entails winding the coil immediately prior to a final heat treating and oxidation step in the fabrication process. This final step results in the repair of micro-cracks incurred during winding, and is used to optimize the superconducting properties of the conductor. However, these results are significantly more difficult to achieve for a coil geometry than for the individual wires which are heat treated and oxidized in the react-and-wind process.
Due to the mechanical properties of the conducting material, superconducting magnetic coils fabricated using the wind-and-react approach with mono-filamentary composite conductors have limitations related to winding density and current-carrying ability. Although the wind-and-react process may repair strain-induced damage to the superconducting material incurred during winding, the coils produced are not mechanically robust, and thermal strain resulting from cool down cycles can degrade the coil performance over time.
A feature of the invention is a wind-and-react process which is used to manufacture superconducting magnetic coils with multi-filament composite conductors. This processing method can be used to manufacture several variations of coils types, all of which are discussed below.
An advantage of the invention is ability to produce mechanically robust coils requiring high winding densities, without significantly degrading the superconducting properties of the multi-filament composite conductors used to form the coils.
The present invention relates to a wind-and-react processing method used to fabricate superconducting magnetic coils featuring strain-tolerant multi-filament composite conductors. This invention has various aspects which individually contribute improvement over previous react-and-wind coils, and wind-and-react coils made with mono-filament conductors. Specifically, materials and processing steps have been adapted in order to fabricate coils which allow adequate oxygen access to the precursor to the multi-filament composite conductor in order to affect conversion to the desired superconducting state, while at the same time allowing preservation of the materials and geometrical tolerances of the coil. Superconducting coils requiring high-density complex winding geometries can often only be fabricated with multi-filament composite conductors because mono-filament conductors are intrinsically less flexible and their electrical properties are more difficult to rehabilitate.
In one aspect, the invention relates to a method for producing a superconducting magnetic coil featuring the following steps: fabricating a precursor to a multi-filament composite conductor from multiple high-temperature superconducting filaments enclosed in a matrix-forming material; surrounding the precursor to the multi-filament conductor with an insulating layer or a precursor to an insulating layer; forming the precursor to the multi-filament composite conductor as a coil; heat treating the coil after the forming step by exposing the coil to high temperatures in an oxidizing environment, the superconductor precursor filaments being oxidized and the matrix-forming material reversibly weakening during the heat treating step, with the composition and thickness of the insulating layer or precursor to the insulating layer being chosen to encase the matrix-forming material and the superconductor precursor filaments, and to permit exposure of the superconductor precursor filaments to oxygen during the heat treating step. The heat treating step results in the improvement of the electrical and mechanical properties of the superconductor precursor filaments, and in the formation of a superconducting magnetic coil.
By xe2x80x9csurroundingxe2x80x9d the eventual multi-filament composite conductor with an insulating layer (or precursor to an insulating layer), direct contact between adjacent conductors is prevented. By xe2x80x9cencasingxe2x80x9d the matrix-forming material and the superconducting precursor filaments during the heat treating step, the insulation layer (or precursor to the insulation layer) preserves the integrity of the coil during the heat treatment. By xe2x80x9creversibly weakeningxe2x80x9d the matrix-forming material is left essentially without mechanical strength during the heat treating step, with the material substantially regaining mechanical stability following processing.
Preferably, the heat treating step involves heating and then cooling the coil in an environment comprising oxygen, and results in the conversion of the superconductor precursor filaments to a desired superconducting material, and in the repair of micro-cracks formed in the filaments during the forming step.
In preferred embodiments, the heat treating step features heating the coil from room temperature at a rate of about 10xc2x0 C./min. until a temperature between 765xc2x0 C. and 815xc2x0 C., and preferably 787xc2x0 C. is obtained; heating the coil at a rate about 1xc2x0 C./min. until a maximum temperature between 810xc2x0 C. and 860xc2x0 C., and prefably 830xc2x0 C., is obtained; heating the coil at the maximum temperature for a time between 0.1 and 300 hours, and preferably for 40 hours; cooling the coil at a rate of about 1xc2x0 C./min until a temperature between 780xc2x0 C. and 845xc2x0 C., and preferably 811xc2x0 C., is obtained; heating the coil at this temperature for a time period in the range of 1 to 300 hours, and preferably for 120 hours; cooling the coil at a rate of about 5xc2x0 C./min. to a temperature between 765xc2x0 C. and 815xc2x0 C., and preferably 787xc2x0 C.; heating the coil at this temperature for a time period between 1 and 300 hours, and preferably for 30 hours; and, finally cooling the coil at a rate of about 5xc2x0 C./min. until a temperature of 20xc2x0 C. is reached, with the heat treating steps performed in an atmosphere which consists primarily of gaseous oxygen at a pressure of about 0.001 to 1 atm, and preferably at 0.075 atm.
In one preferred embodiment of the invention, the coil is formed by repeating the steps of first winding a layer of the precursor to the multi-filament composite conductor around a mandrel, and then winding a layer of material comprising an insulating material or a precursor to an insulating material on top of the precursor to the multi-filament composite conductor. In another preferred embodiment of the invention, the precursor to the insulating material is initially a liquid mixture of a solvent and dispersant, and a particulate material, with the mixture being applied by dipping the precursor to the multi-filament composite conductor in the liquid mixture, followed by a heating step which results in the evaporation of the solvent and dispersant, and the formation of an insulating layer around the precursor to the multi-filament composite conductor. In a preferred embodiment of the invention, a heating step is used to remove impurities from the insulating material, such as dirt or a binder material.
In another preferred embodiment of the invention, the coil forming step features the step of concentrically winding the precursor to the multi-filament composite conductor to form a multi-layer coil having a xe2x80x9cpancakexe2x80x9d shape, with each of the layers wound to overlap the preceding layer. Each edge of the entire length of the precursor to the multi-filament composite conductor in this geometry is exposed to the oxidizing environment during a heat treating step. The heat treatment results in the oxidation and healing of micro-cracks in the superconductor filaments of the precursor to the multi-filament composite conductor, resulting in the formation of a multi-filament composite conductor. The xe2x80x9cpancakexe2x80x9d coil can be wound around a mandrel having an arbitrary shape. In preferred embodiments, the xe2x80x9cpancakexe2x80x9d coil is wound around a mandrel having a circular cross section. In alternate embodiments, the mandrel cross section is primarily elliptical in shape. In preferred embodiments, double xe2x80x9cpancakexe2x80x9d coils can be formed by winding a second xe2x80x9cpancakexe2x80x9d coil on the mandrel adjacent to the first xe2x80x9cpancakexe2x80x9d coil. In yet other preferred embodiments of the invention, multiple double xe2x80x9cpancakexe2x80x9d coils can be combined to form a single coil, and are preferably stacked in a coaxial manner.
In one particular aspect of the invention, a method for producing a superconducting magnetic coil, similar to the method described above, features subjecting the precursor to the multi-filament composite conductor to a bend strain in excess of its critical strain. In a particular embodiment of the invention, the precursor to the multi-filament composite conductor is subjected to a bend strain in excess of 0.3%.
In another particular embodiment, each layer of the multi-filament composite conductor of the coil consists of multiple conductors, with all of the conductors surrounded by a single insulating layer. Preferably, the multi-filament composite conductor has multiple superconducting filaments enclosed in a matrix-forming material composed of a noble metal or an alloy to a noble metal, and is preferably made of silver. In a particular embodiment, the superconducting material used for the filaments is selected from the oxide superconductor family, comprising the following materials: (Bi,Pb)2Sr2Canxe2x88x921CunO2n+4, where n is equal to either 1, 2, or 3; members of the rare earth-copper-oxide family, such as YBCO (123), YBCO (124), and YBCO (247); members of the thallium-barium-calcium-copper-oxide family, such as TBCCO (1212) and TBCCO (1223); and, members of the mercury-barium-calcium-copper-oxide family, such as HgBCCO (1212) and HgBCCO (1223). Preferably, three-layer phase BSCCO is used for the superconducting filaments.
In preferred embodiments of this aspect of the invention, the multi-filament composite conductor is surrounded by an insulating layer which is permeable to gaseous oxygen and substantially chemically inert relative to the multi-filament composite conductor. In a preferred embodiment, an insulating material selected from the group containing SiO2, Al2O3, and zirconia fibers is used as the insulating layer. Preferably, the insulating material is co-wound with the precursor to the multi-filament composite conductor. In alternate embodiments, the insulating material is wrapped around the precursor to the multi-filament composite conductor. Preferably, the thickness of the insulating layer is between 10 and 150 m. In other embodiments, the insulating layer of the coil consists primarily of a particulate material selected from a group comprising Al2O3, MgO, SiO2, and zirconia.
In particular aspects of the invention, a superconducting magnetic coil made with the method described above has an inner-coil diameter no larger than about 1 cm, or alternatively, the coil is wound so that the bend strain of the multi-filament composite conductor is greater than 0.3%. In other aspects of the invention, the winding density of the coil is greater than about 60%, the fill factor of the multi-filament composite conductor is greater than about 30%, the minimum critical-current is about 1.2 Amperes, and the magnetic field produced by the coil is in excess of about 80 Gauss.
In one aspect of the invention, a xe2x80x9cpancakexe2x80x9d coil is formed by the method described above. In a preferred embodiment, each layer of insulated multi-filament composite conductor of the xe2x80x9cpancakexe2x80x9d coil consists of multiple strands of multi-filament composite conductor, each having multiple superconducting filaments, with all strands surrounded by a single insulation layer. The conducting and insulating materials used in the xe2x80x9cpancakexe2x80x9d coil are the same as those described previously. In one embodiment of the invention, the coil is impregnated with a polymer. In a preferred embodiment, double xe2x80x9cpancakexe2x80x9d coils can be formed. Double xe2x80x9cpancakexe2x80x9d coils can be stacked coaxially and adjacent to each other. In certain preferred embodiments, the mandrel supporting the stacked coils is removed.